1. Field of the Invention
The present invention relates to a semiconductor device having circuits structured by electric field effect transistors (FETs), for example, thin film transistors (TFTs), and to a method of manufacturing the semiconductor device. The present invention relates, for example, to a semiconductor device, typically a liquid crystal display panel, and to an electronic device in which such a semiconductor device is mounted as its component.
Note that, throughout this specification, the term electro-optical device indicates general devices for performing shading display by changing an electrical signal, and that liquid crystal display devices and display devices using electroluminescence (EL) are included in the category of electro-optical device.
Note also that, throughout this specification, the term element substrate indicates general substrates on which active elements such as TFTs and MIMs are formed.
2. Description of the Related Art
Techniques of structuring thin film transistors using semiconductor thin films (having thicknesses on the order of several nm to several hundred nm) formed on a substrate having an insulating surface have been focused upon in recent years. The thin film transistors are being widely applied to electronic devices Like ICs and semiconductor devices, and in particular, their development has accelerated rapidly as switching elements of liquid crystal display devices.
Liquid crystal display devices are known to be roughly divided into active matrix types and passive matrix types.
A high-grade image can be obtained with active matrix liquid crystal display devices using TFTs as switching elements. Active matrix applications are generally to notebook type personal computers, but they are also expected to be used in televisions for a home and in portable information terminals.
The active matrix liquid crystal display devices are generally driven by line inversion drive. With the line inversion drive, for example source line inversion drive, the polarity of voltages applied to adjacent source lines differs, as shown in FIGS. 37A and 37B, and the polarity of the voltage applied to each source line changes each frame. FIGS. 37A and 37B show the polarity of voltages applied to pixels during the source line inversion drive. Drive in which the polarity of the voltage differs for each adjacent source line is referred to as the source line inversion drive. Drive in which the polarity of the voltage differs for each adjacent gate line is referred to as gate line inversion drive.
FIG. 10 shows schematically a cross section of a pixel portion of a liquid crystal display device. An electric field formed between pixel electrodes 102a and 102b formed on a substrate 101, and an opposing electrode 103 formed on an opposing substrate 104, as shown in FIG. 10, is referred to as a vertical direction electric field 105 in this specification. Further, an electric field formed between the adjacent pixel electrodes 102a and 102b is referred to as a horizontal direction electric field 106 in this specification.
Liquid crystals in the vicinity of the pixel electrodes orient themselves along the horizontal direction electric field if the line inversion drive is performed, the liquid crystal orientation in edge portions of the pixel electrodes becomes nonuniform, and disclinations develop. In order to obtain a good quality black level, light shielding films for covering the disclinations are necessary. However, the aperture ratio drops if the disclinations are covered by the light shielding films. It is necessary to come up with a scheme in which a good quality black level can be obtained, and as little disclination as possible develops when displaying a high aperture ratio, bright image. Note that, in this specification, liquid crystal orientation irregularities developing due to differences in the direction of the pre-tilt angle, and differences in the twist direction, at liquid crystal orientation film interfaces are referred to as “disclinations”. Further, regions having different brightnesses produced due to an irregular orientation state of the liquid crystals when a polarization plate is formed is referred to as “light leakage”.
In particular, the occupied ratio of pixels in which disclinations and light leakage developing due to horizontal direction electric fields is large enough that it cannot be ignored in liquid crystal display devices in which the pixels are formed at a very fine pitch, such as that of a projecting liquid crystal display device. Further, these disclinations and light leakage are expanded when projected onto a screen with the projecting liquid crystal display device, and therefore whether or not the light leakage and disclinations can be suppressed is vital in maintaining contrast.